Geoscience Reference
In-Depth Information
of the IPCC's TAR and the subsequent Fourth
Assessment Report (AR4), released in 2007.
For the SRES families of scenarios, less CO
2
is
emitted in the more ecologically friendly B scenar-
ios, with B1 being for a more integrated future
world and B2 being associated with a more divided
future world having higher emissions. More CO
2
is
emitted in the non-ecologically friendly A family of
scenarios, with A2 being a divided future world
and A1 being more integrated. The latter set was
further divided by weighting the different energy
types: A1T (non-fossil-fuel emphasis), A1B ('bal-
anced'), and A1FI (fossil-fuel intensive). Because
modelling groups cannot usually afford to run all
scenarios, they often pick a low scenario such as B1
and a high scenario such as A2 in order to bracket
the others.
The IPCC has also used concentration scenarios
or pathways to investigate processes under a pre-
set stabilization goal for atmospheric greenhouse
gas concentrations. Further discussion about these
and the latest generation of scenarios that will
be used in the next IPCC report is provided in
Chapter 14 .
pogenic nitrogen and sulphur, which can exacerbate
acidii cation of coastal waters (Doney
et al.
2007 ).
Both approaches also neglect buffering effects from
the dissolution of CaCO
3
sediments, which would
increase alkalinity but is negligible over centuries
for shallow sediments and over millennia for deep
sediments (see Chapter 7). Below, these approaches
are detailed and their projections discussed.
3.6.1 Approaches to project future
acidii cation
The equilibrium approach computes future surface-
ocean pH and [CO
3
2-
] using basic thermodynamic
equilibrium equations while varying CO
2
and hold-
ing constant another carbonate system variable,
typically
A
T
. Conveniently, this approach does not
rely on an ocean model. It is exact when the inde-
pendent variable is seawater
p
CO
2
, not atmospheric
p
CO
2
. In other words, it assumes thermodynamic
equilibrium between CO
2
in the atmosphere and
that in surface waters at their
in situ
A
T
, tempera-
ture, and salinity. The equilibrium approach works
well in regions such as the subtropical gyres where
waters remain at the surface long enough for
C
T
to
equilibrate with atmospheric CO
2
, typically requir-
ing 8 months at present (see Box 3.1); conversely,
the equilibrium approach is less accurate in areas
such as the tropical Pacii c or high latitudes, where
waters spend less time at the surface and thus have
insufi cient time to equilibrate with the atmosphere.
The equilibrium approach is also inappropriate for
the deep ocean, which is isolated from the
atmosphere.
When calculating future changes in surface car-
bonate system variables, the equilibrium approach
is inaccurate wherever the anthropogenic transient
of
p
CO
2
in the surface ocean lags that in the atmos-
phere. The assumption of equilibrium with the
atmosphere leads to the prediction that a given
reduction in pH or saturation will occur too soon, at
lower atmospheric
p
CO
2
. For example, Orr
et al.
(2005) demonstrated that the equilibrium approach
predicts that average surface waters of the Southern
Ocean become undersaturated with respect to arag-
onite, as discussed below, when atmospheric
p
CO
2
is 550 ppmv (in 2050 under the IS92a scenario); con-
versely, their disequilibrium approach, relying on a
3.6 Projecting future changes in
carbonate chemistry
Under all future scenarios in which atmospheric
CO
2
increases, it is well known that ocean acidii ca-
tion will intensify. This basic conceptual under-
standing is backed up by a simple approach that
uses well-known fundamental thermodynamic
equations to quantify future changes by assuming
equilibrium between atmospheric and surface-
ocean CO
2
. This equilibrium assumption works
well over most of the surface ocean, i.e. where the
air-sea CO
2
equilibration timescale (several months;
see Box 3.1) is much shorter than the residence time
of waters near the surface (Sarmiento and Gruber
2006). A second approach relying on global-scale
ocean models coni rms this future intensii cation of
ocean acidii cation and provides a more realistic
regional picture by accounting for air-sea CO
2
dis-
equilibrium, such as is found in regions where there
is substantial exchange between surface and deep
waters. Neither approach considers the effects of
eutrophication or atmospheric deposition of anthro-
